My 1993 Chevy C1500 with 4L60E has had a somewhat annoying buzz for a while now. It only occurs when in a drive gear (not park or neutral) and only when under no/light throttle. This is a fairly common issue as I've heard it on other trucks but that doesn't help me figure out what's going on. I can only describe it as a loud buzzing (heard from inside the cab with the windows down) coming from the transmission. It doesn't vary with speed but as soon as you go beyond extremely light throttle will instantly disappear. Any ideas? The transmission currently has about 25,000 miles on it and operates properly.